Page 652 Fourth International Symposium on Space Terahertz Technology BROADBAND QUASIOPTICAL SIS MIXERS WITH LARGE AREA JUNCTIONS G. Pance and M. J. Wengler Department of Electrical Engineering University of Rochester Rochester, NY 14627 Abstract We have designed and tested a broadband quasi-optical superconducting tunnel junction (SIS) mixer with integrated tuning elements. We achieve state of the art results using low critical current density, large area niobium SIS's which are commercially available from Hypres, Inc. We have performed noise measurements in the frequency range from 70 GHz to 105 GHz. The best uncorrected double sideband receiver noise is 38 K at 77 GHz, with receiver noise temperatures less than 100 K from 75 to 102 GHz. I. Introduction Superconducting tunnel junctions (SIS's) are used in the most sensitive detectors at frequencies from 100 GHz to 750 GHz [1]. Much of the recent progress in improving sensitivity has relied on using very small area (submicron) junctions with very high critical current densities exceeding 10 kA/cm 2 [2-5]. The large current density allows higher frequency operation of junctions, while the small junction area is necessary to match large current density junctions to moderately high impedance radiation structures. Unfortunately, submicron lithography is very expensive and not widely available, so it is time consuming and expensive to develop submicron circuits. Similarly, 1 kAicm 2 SIS technology is well developed for digital circuit research, but higher current densities require device development separate from the larger digitally oriented efforts. We show in this paper that large area, low current density SIS's can be used to make state of the art SIS mixers. We do this by designing our mixers for Hypres, Inc.'s standard niobium integrated circuit process. We use inductive tuning to cancel out the large capacitance of large area SIS's, and a transformer to match to high impedance antennas. We are thus able to take advantage of all of the technology development effort that has gone into digital circuits. II. Circuit Design and Simulations In our quasi-optical SIS receiver, the SIS junction is built integrally with a planar self-complementary log-periodic antenna on the silicon substrate [6-8]. The antenna is placed on the back of a quartz hyperhemisphere [9]. The hyperhemisphere and a teflon lens in front of it focus the radiation onto the antenna. The antenna impedance is frequency independent over several octaves and is around 7611 Tuning structures are built on one arm of the antenna which is used as their ground plane (Fig. 1). The major goal in the design of integrated tuners is a large bandwidth of good coupling. A large bandwidth design has two advantages. First, it provides a sensitive